Marcasite (FeS₂) is the orthorhombic dimorph of pyrite (cubic FeS₂) — same chemistry, different crystal structure. It forms in sedimentary low-temperature environments (coal seams, organic-rich shales) and oxidizes more readily than pyrite, producing characteristic “pyrite disease” disintegration in collections. Cockscomb spear-cluster habits are diagnostic.
Key Facts
- Mohs hardness 6–6.5.
- Orthorhombic; dimorph of pyrite (cubic).
- Pale brass-yellow with greenish tarnish.
- “Cockscomb” twin clusters and tabular crystals are diagnostic.
- Unstable — undergoes “pyrite disease” decomposition in humid conditions.
Notable Localities
Galena (Illinois, USA) and Folkestone (England) yield classic specimens. Joplin tristate Pb-Zn district produces cockscomb clusters.
Found at these Localities
- Rapid Creek (Yukon) (育空地区急流溪磷酸盐产地)
- Tri-State Mining District (Joplin) (三州矿区(乔普林))
